Dredging is typically done with vessels, such as barges or ships which have spuds, which are driven into the water bed at one end of the vessel while a boom or ladder at the other end of the vessel extends beneath the water to the bed from which the solids are to be removed. Often, at the lower end of the ladder, there is a cutter which breaks loose the bed and forms particles for suction as suspended solids into a pipe having a suction inlet at the lower end of the ladder. The solids are pumped up the ladder by a centrifugal impeller pump and the solids are loaded onto the vessel or in some instances pumped through pipes or hoses to a location on shore for deposit. The centrifugal pumps are often located on the vessel and are driven by an internal combustion through a speed-reducer drive unit between the engine and the centrifugal impeller pump. The speed-reducer unit reduces the output speed of the engine often above 1200 r.p.m. and as high as several thousand r.p.m. to 400-600 r.p.m. which is the usual most efficient speed of such pumps. These pumps can not be driven at the higher engine speeds without cavitation. The speed-reducer units are heavy and costly.
Current centrifugal impeller pumps are very heavy users of fuel and are very heavy. By way of example, an 14-inch inlet centrifugal dredging pump may weigh in the range of 12,500 to 20,000 pounds. The weight of the pumps and their speed reducer drives severely limits the portability of small dredges to be trucked from one site to another site. Typical fuel costs for a small 10-inch centrifugal pump dredge may be about $60 per hour which is about one-half of the total operating costs per hour. The typical flow rates for an 8-inch centrifugal pump may be 100 tons per hour when dredging blue clay and the solids content flowing through the pipes and pump is often in the range of 16% to 17% by weight of blue clay solids. Thus, it will be seen that current dredging is costly in the terms of fuel expended and low in the terms of solids content being pumped. Too much water is being pumped and this is a problem if the dredged material must be trucked to a disposed site or barged out to sea. Thus, there is a need for more efficient dredging operations.
Another problem with current centrifugal pumps is that of the amount of downtime for unclogging the impellers of debris due to the fact the impeller and pump casing have narrow throats defined by portions of the impeller being located closely adjacent a pump casing surface to create the suction for pumping and that debris may be caught in this narrow space. Often, the river or harbor bottoms are full of debris including many parts of automobiles and tires as well as other refuse dumped into the water. Also, sunken logs, tree branches and other materials may become wedged in the centrifugal pump. Additionally, the impellers are exposed directly to solids which often contain sharp articles which crack or break laminated wear surfaces on the impeller or pump casing causing a rapid deterioration or de-lamination, particularly where the solids are highly abrasive sand or gravel. In any event, dredges in rivers or harbors often experience several hours of downtime per day because of impeller clogging or need of repair. The cost of small dredges may range from $125 to $300 per hour so that the amount of money lost to downtime and maintenance is significant. On very large dredges, where twenty or thirty people are on board, the downtime with no production is much more expensive.